This invention relates generally to an electrophotographic printing machine, and more particularly concerns a development apparatus employing a single component magnetic developer material.
In the process of electrostatographic printing, an electrostatic latent image is formed on a surface and reproduced in viewable form. The broad class of electrostatographic printing includes electrophotography and electrography. Electrophotography employes a photosensitive medium to form, with the aid of electromagnetic radiation, an electrostatic latent image on the photosensitive medium. Electrography is that class of electrostatography which utilizes an insulating means to form, without the aid of electromagnetic radiation, the electrostatic latent image. Hereinafter, an electrophotographic printing process will be discussed wherein the single component magnetic developer material is deposited directly onto the sheet of support material by the development apparatus. In electrophotography, a photosensitive element having a photoconductive insulating layer is charged to a substantially uniform potential in order to sensitize its surface. The charged photoconductive surface is exposed to a light image of an original document being reproduced. As a consequence of the exposure, the charge is selectively dissipated in the irradiated areas in accordance with the intensity of light reaching the surface thereof. This forms an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. The electrostatic latent image recorded on the photoconductive surface may then be developed by bringing a developer material into contact therewith. Alternatively, a sheet of support material may be placed in contact with the latent image recorded on the photoconductive surface and the developer material brought into contact with the sheet of support material. In this latter embodiment, the developer material is attracted directly to the copy sheet. Contrawise, in the former embodiment, the developer material must be transferred from the photosensitive member to the copy sheet. In both of the foregoing processes, developer material is heated to permanently affix it to the copy sheet. The broad concept of electrophotographic printing was originally disclosed by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described by many related patents in the art. Exemplary patents describing machines wherein the copy sheet is positioned on the photoconductive member prior to development are the following: U.S. Pat. No. 2,885,955 issued to Vyverberg in 1959, U.S. Pat. No. 3,093,039 issued to Rheinfrank in 1963, U.S. Pat. No. 3,166,432 issued to Gundlach in 1965, and U.S. Pat. No. 3,251,706 issued to Walkup in 1966.
In the foregoing types of printing machines, a development apparatus is employed to deposit developer material onto either the copy sheet or the photoconductive member. Generally, the developer material comprises toner particles, which are mixed with coarser carrier granules. Typical toner particles are made from a thermoplastic material while the carrier granules are made from a ferromagnetic material. Alternatively, single component magnetic particles may be employed.
A system utilizing single component magnetic developer would be capable of high speeds. One type of development apparatus employing a single component magnetic material is described in U.S. Pat. No. 2,846,333 issued to Wilson in 1958. It has been found that when a single component magnetic developer material is employed, the resultant copy has high contrast. However, continuous tone graduations are difficult to obtain. The development process is dependent upon the spacing between the donor roll and photoconductive surface. Thus, changes in the spacing between the photoconductive surface and donor roll or copy sheet and donor roll will effect the resultant contrast. Various types of donor rolls have been developed which have irregular surfaces. For example, U.S. Pat. No. 3,246,629 issued to Shelfo discloses a flame spray used to provide a layer of irregular shaped particles which adhere to the exterior circumferential surface of the developer roll providing a randomly roughened surface. U.S. Pat. No. 3,863,603 issued to Buckley et al. describes a magnetic brush roller having a resilient roughened polyurethane coating thereon. Similarly, U.S. Pat. No. 3,176,652 issued to Mott describes a magnetic brush apparatus having an elongated magnet held stationarily in a rotating shield. The shield may be plastic with the outer surface thereof roughened in a random or rectangular pattern. Finally, U.S. Pat. No. 3,563,734 issued to Shely discloses a developer powder applicator made from an roller coated with a conductive rubber or hardened conductive gelatin. The roller may be a rotatably mounted non-magnetic metal cylinder containing a stationary permanent magnet therein.
However, none of the prior art patents appear to disclose a donor member having a screen pattern thereon in order to vary the spacing between the photoconductive member and donor member so as to form a continuous gray scale during the development process.
Accordingly, it is a primary object of the primary invention to improve the donor roll used in the development apparatus employed of an electrostatographic printing machine.